1. Field of the Invention
The present invention generally relates to optical encoders. Particularly, the present invention relates to optical encoders with enhanced resolution for measuring angles and displacements.
2. Description of the Related Art
An optical encoder includes a main scale having a first optical grating, and an index scale having a second optical grating. Disposed opposite the main scale is a light source for irradiating the main scale with light, and a photoreceptor element that receives light via the optical grating of the main scale and the optical grating of the index scale. A photoreceptor element array that also functions as an index scale has been used in this type of optical encoder.
FIG. 9 is a schematic diagram showing the construction of a conventional photoelectric encoder. As shown in FIG. 9, a detecting-side grating substrate 232 includes photoreceptor elements 258 disposed at a regular pitch. As shown in FIG. 10, each of the photoreceptor elements 258 includes a first signal lead-out layer 252 composed of a light-blocking, conductive material such as a metal film, a PN semiconductor layer 254 that converts light into an electric signal, and a second signal lead-out layer 256 composed of a light-transmitting, conductive material such as In2O3, SnO2, Si, or a mixture thereof, laminated in that order on a light-transmitting base 250 composed of, for example, glass. The photoreceptor elements 258 are disposed opposite a main scale 224, and the photoreceptor elements 258 form slits.
Light reaches the PN semiconductor layer 254 via the second signal lead-out layers 256 of the photoreceptor elements 258, and is photoelectrically converted at the boundary between an N-type amorphous silicon film 260 and a P-type amorphous silicon film 262. The resulting signals are extracted to the outside from output terminals 264 and 266.
In this type of optical encoder, a light-emitting-side grating substrate 230 is formed integrally with light-emitting elements 212, and the detecting-side grating substrate 232 is formed integrally with the photoreceptor elements 258, serving to reduce the number of parts and to reduce size and weight.
FIG. 11 shows the relationship between an example of a pattern of photoreceptor photodiodes in the encoder and a pattern of detected bright and dark light. Photodiodes S1 to S4 are repeatedly subject to signal phase shifts of 0°, 90°, 180°, and 270° with respect to a bright and dark pattern of light represented by a sine wave. Signals generated by the photodiodes S1 to S4 are input to a current-voltage converter circuit (not shown). Then the signals from the current-voltage conversion are shifted by 90° with respect to each other. By differential amplification, analog sine-wave voltage signals of two phases are obtained. For example, phase A (S1–S3) and phase (S2–S4) having phases of 0° and 90°, are obtained.
Actually in the encoder, the analog sine-wave voltage signals are input into a comparator, and the resulting digital signals are fed into a counter circuit or the like.
In a conventional encoder, in order to further enhance resolution, the pitch of the scale and the bright and dark regions of the photoreceptor elements must be further reduced.
However, when the scale pitch is reduced, a considerable decrease in precision occurs. This is because the amplitudes of signals obtained by the photoreceptor elements become smaller, causing noise or affecting the hysteresis of the comparator used for digitization, resulting in a considerable decrease in precision.